Light conversion (or wavelength conversion) materials such as phosphors are used in a variety of applications, especially in optical devices. One such application is a phosphor-in-silicone colour wheel, which is an optical device for generating emission light of one or typically multiple different wavelengths from excitation light of a single wavelength. An example phosphor-in-silicone colour wheel is described in WO-2014/016574, having common inventorship. In such a phosphor-in-silicone colour wheel, the phosphor powders are dispersed in matrices of liquid transparent silicone and then thermally cured and solidified.
An example, known phosphor wheel structure of this type is shown in FIG. 1. This structure is a phosphor-in-silicone-glue reflective colour wheel. A light converter (in this case, a single colour phosphor-silicone ring) 101 is provided on a metal disc substrate with mirror surface 102. Excitation light 103a (which may also be called source light, input light or exciting light) e.g. from a laser-based illumination source or other light source (not shown) causes the generation of emission light 103b (also called emitting/converted reflected light), when it is incident on the light converter 101. The light converter 101 converts the light spectrum from excitation light of a first range of spectral wavelengths to emission (or re-emission) light of a second, different range of spectral wavelengths. When the excitation laser beam 103a (for example, blue light) focuses on the phosphor ring 101, the conversion light beam 103b (for example yellow light) will emit and will be reflected by the mirror coating metal disk 102, and then will be collected by a lens system. The wheel is comprised of a single colour phosphor ring 101 or multiple colour segments (not shown here), each of which is used to generate light with a particular colour. The phosphor powders are dispersed in liquid transparent silicone by dispensing or screen printing, and then thermally cured and solidified to colour segments on a mirror coating metal disk 102 in a concentric pattern. Then, the colour wheel is mounted on a motor to rotate with high speed. Typically, the disc substrate 102 is rotated during use, although this device can be used in a static (non-rotating) configuration, in which case it may not be known as a phosphor wheel. The light converter 101 is conventionally formed as a coating, comprising phosphor particles in a polymer binder.
However, for higher power applications, undesirable temperature effects have been observed in the conversion material. In an approximately 100 W laser projector, with the excitation laser focused on the phosphor-in-silicone-glue colour wheel, the temperature of the laser conversion area will be over 200 degrees Celsius. At high temperatures, the light conversion efficiency will drop sharply (>10% @ 200 degrees Celsius) due to thermal quenching. Additionally, the silicone glue will degrade seriously with intense laser irradiance and gradually the glue will be burnt. Thus, the phosphor-in-silicone-glue colour wheel cannot achieve a long operational life in high power laser projectors. In life-time tests for such a product, it was established that the safe working temperature should be controlled under 150 degrees Celsius.
It is therefore desirable to create an optical light conversion device that is operable with high-power sources, for example laser projectors, without significant reduction in efficiency or working lifetime.